Smart completion
Ctrl+Shift+Space gives you a list of the most relevant symbols applicable in the current context. This and other completions are constantly learning from you, moving the members of the most frequently used classes and packages to the top of the suggestions list, so you can select them faster.

Chain completion
Digs a tad deeper than Smart Completion and lists applicable symbols accessible via methods or getters in the current context. Say you're looking for a value of Project and only have the Module module declaration. Press Ctrl+Shift+Space twice to get module.getProject() without any additional effort.

Language injection
Brings coding assistance for a selected language to expressions and string literals in another one, complete with all advantages you would normally have. For example, you can inject fragments of SQL, XPath, HTML, CSS, or JavaScript code into Java String literals.

Cross-language refactorings
Knowing everything about usages of a symbol, IntelliJ IDEA offers extremely effective, thorough refactorings. For example, when you Rename a class within a JPA statement, it will update everything, from JPA entity class, to every JPA expression where it is used.

Detecting duplicates
Finds duplicate code fragments on the fly. Even if you're only about to extract a variable, constant, or a method, IntelliJ IDEA will let you know that there is a similar code fragment that can be replaced along with the one you're woking on.

Inspections and quick-fixes
Whenever IntelliJ IDEA detects that you're about to make a mistake, a little lightbulb pops up in the editor. Clicking it or pressing Alt+Enter opens a list of actions you can take to make things right.

Developer ergonomics
Every aspect of IntelliJ IDEA is designed with ergonomics in mind. IntelliJ IDEA is built on a principle that every minute a developer spends in the flow is a good minute, and things that break developers out of that flow are bad and should be avoided.

Built-in developer tools
To streamline your workflow, IntelliJ IDEA offers an unbeatable toolset right from the first start: decompiler, bytecode viewer, FTP and many more.

Polyglot experience
In addition to Java, IntelliJ IDEA offers first-class support for top JVM and non-JVM frameworks and languages out of the box.